In a typical electrophotographic printer, a latent image charge pattern is formed on an electrostatic imaging member in accordance with an image to be printed and the electrostatic image is developed with charged toner particles. The charged toner particles adhere to the latent image charge pattern on the electrostatic imaging member to form a toner image. The toner image is then transferred from the electrostatic imaging member to a transfer subsystem and from the transfer subsystem to a receiver. The toner and receiver are then fused to form a print.
In certain circumstances, less than all of the toner forming the toner image transfers from the electrostatic imaging member to the transfer system. This leaves residual toner on the electrostatic imaging member that can create unwanted artifacts in subsequent toner images formed on the electrostatic imaging member. Additionally, other material such as fuser oil, coatings and fragments of toner particles, agglomerates, carrier, paper fibers, paper coatings, dirt, dust and other charged materials in the environment surrounding the printer can be attracted to and can accumulate on the electrostatic imaging member to form a layer. This layer can be difficult to remove and can also cause unwanted artifacts in subsequent toner images formed on the electrostatic imaging member. Accordingly, electrostatic imaging members are typically cleaned between or within image printing cycles to remove any such residual toner and other material (referred to herein collectively as “residual material”).
Various techniques have been developed to clean electrostatic imaging members. In some devices, magnetic or electrically biased members are used to attract residual material from an electrostatic imaging member (see for example U.S. Pat. No. 4,639,124 issued to Nye, Jr. et al. on Jan. 27, 1987.) In other devices, cleaning is performed using a fabric or other type of contact brush (see for example U.S. Pat. No. 4,999,679 issued to Corbin et al. on Mar. 12, 1991). Such brushing techniques, while generally effective at removing residual toner have proven less effective at removing the other types of residual material.
Accordingly, other types of cleaning systems have been developed to try to remove such residual material. One type of cleaning system is a scraping system in which a blade is held with a working face that extends toward an electrostatic imaging member in a direction that opposes the direction of movement of the electrostatic imaging member. In such systems, residual material is scraped from the electrostatic imaging member as the electrostatic imaging member is moved past the blade.
One example of a scraping system is U.S. Pat. No. 3,947,108 issued to Thettu et al. on Mar. 30, 1976. In the '108 patent, a blade is shown that oscillates back and forth across a drum during cleaning. The blade has a leading edge in contact with a surface of the drum. The blade is positioned so that the blade extends toward the drum in a direction opposite to a direction of drum rotation to shear material from the face of the drum. However, in the '108 patent, the blade is used to remove residual toner particles so as make a secondary brush cleaner more efficient at removing a film of other material from the drum.
In U.S. Pat. No. 4,989,047 issued to Jugle et al. on Jan. 29, 1991, a thin scraper member is provided as a secondary cleaner to remove agglomerations of toner and debris from an electrostatic imaging member after a cleaning brush has had an opportunity to clean the electrostatic imaging member. FIG. 1, which is adapted from FIG. 2B of the '047 patent, shows one embodiment of a thin scraper 300 that extends from a holder 302 toward an electrostatic imaging member 304 in a direction 306 that is the opposite of a direction of movement 308 of the electrostatic imaging member 304.
As is also shown in FIG. 1 scraper 300 extends from holder 302 at a first angle 310 and contacts electrostatic imaging member 304 at a shallow working angle 312. This approach advantageously allows scraper 300 to provide a substantial amount of cleaning force FC against any residual materials on electrostatic imaging member 304 while applying only a limited amount of normal force FN against electrostatic imaging member 304. A very low scraping angle is used, for example between just over 0 and up to 9 degrees and a load is applied to help keep the scraping blade against the surface being cleaned.
However, scraping systems are subject to a failure mode known as blade tuck or “tuck under”. FIG. 2 shows an example of this condition in the context of the scraper shown in FIG. 1. As is shown in FIG. 2, a blade tuck occurs when a leading edge 314 of a scraper 300 folds under scraper 300. Blade “tuck” can happen because, for example, the frictional force between leading edge 314 and electrostatic imaging member 304 reaches a high enough level to cause leading edge 314 to move with electrostatic imaging member 304.
A tucked under scraper 300 creates a normal force FN against the electrostatic imaging member 304 that can be substantially greater than the normal force FN of scraper 300 in a normal state and provides substantially reduced cleaning force FC. This can create wear marks and scratches on the electrostatic imaging member 304, reduce the useful life of scraper 300 and the electrostatic imaging member 304 as well as interrupting work flow and wasting consumables.
In embodiments described in the '047 patent the blades are mounted in a movable mountings that allow the scraping blades to be moved in the vertical direction and a low load is placed on the blades so that a maximum shearing force can be applied by the blade. This is done to avoid the problems associated with normal cleaning engagement of blades with a charge retentive surface. According to the '047 patent, because of the low load of the blade, the minimal amount of toner that normally passes through any cleaning system serves as a lubricant for the blade without the need for further added lubricant.
U.S. Pat. No. 5,031,000, issued to Pozniakas et al. which is a continuation in part from the application leading to the '047 patent, provides claims that are directed to a blade supported in a floating support assembly. The blade floats under a low weight during break in of a new blade to prevent tuck under and damage to the blade. The weight applied to the blade is optimized for the break in period and the support assembly has a stop to prevent blade creep during normal operations.
U.S. Pat. No. 5,349,428, issued to Derrick on Sep. 20, 1994, also notes that the leading edges of scraping blades are subject to a failure mode known as blade “tuck”. The '428 patent proposed to solve this problem using a variable position drum.
Because scrapers oppose the direction of motion of the electrostatic imaging member another problem that can arise with the use of a scraper is the so called “chatter” problem. Chatter occurs because the coefficient of static friction between the scraper and the electrostatic imaging member is greater than the coefficient of dynamic friction between the scraper and the electrostatic imaging member. Accordingly, when movement of the electrostatic imaging member is slow the coefficient of static friction can cause the scraper to deflect in the direction of motion of the electrostatic imaging member until sufficient elastic energy is stored in the scraper to allow the scraper to overcome the static friction causing rapid movement of the cleaning edge of the scraper. This rapid movement reduces cleaning efficiency and creates bands of uncleaned or partially cleaned areas on the electrostatic imaging member.
Alternatively it has been known to clean an electrostatic imaging member using a wiper. FIG. 3 illustrates one example of a wiper type cleaning system 318. In this example, wiper 320 is held by a holder 322. Holder 322 extends toward electrostatic imaging member 304 in a direction 324 of movement of electrostatic imaging member 304. Because such wipers extend toward the electrostatic imaging member 304 in the direction of movement of the electrostatic imaging member, wiper type cleaning systems are not subject to the blade “tuck” failure mode that occurs with scrapers. Wiper cleaning systems 318 however have working angles 326 that are higher than the working angles used in scraper systems. For this reason wiper cleaning systems 318 typically apply a greater amount of normal force FN against the electrostatic imaging member 304 being cleaned to achieve a desired cleaning force FC than do scraper systems. This can increase the amount of friction acting on an electrostatic imaging member 304 and can impact the useful life of the electrostatic imaging member 304 and wiper 320. Such results can become particularly pronounced where a high cleaning force FC is required.
The working angle 326 of the wiper 320 is established as a function of holding angle 328 at which wiper 320 is held and the free length L of wiper 320 when unbent (shown in phantom in FIG. 3), and a variety of factors including the separation distance 325 between holder 322 and electrostatic imaging member 304. Ultimately, the holding angle 328 determines the highest possible working angle 328 for a wiper, with other factors controlling the extent to which the working angle 326 will deviate from holding angle 328.
It will be appreciated that in a wiping system such as wiping system 318 there can be variations in these factors and that wiping system 318 will be defined in a manner that provides a minimum cleaning force FC at all possible working angles 326 within the range of variability in these factors. This typically requires that wiping system 318 provides this minimum cleaning force FC over a wide range of working angles 326. When wiping system 318 is operated at low working angles 326 in the range, the amount of normal force FN that must be applied to the electrostatic imaging member 312 to achieve the minimum desired cleaning force FC increases significantly.
What is needed therefore is a cleaning solution that removes residual materials from an electrostatic imaging member and that also does so with limited normal force, reduced chatter and reduced risk of blade “tuck” incidents.